Many technological and scientific application can benefit from compact sources of soft x-ray and/or extreme ultraviolet laser radiation. Such radiation is made of energetic quanta (photons) having an energy much larger than that of visible light quanta. Consequently, it can interact with matter in different ways than visible or ultraviolet light, therefore inducing reactions or recording features that would not be possible with longer wavelength radiation.
A few technologies have been developed to produce such beams of soft x-ray and/or extreme ultraviolet radiation. They include synchrotron sources and soft x-ray lasers. These devices are, however, expensive and very large in size and complex. These limitations put those powerful sources out of the reach of most individual and small user groups.
Soft x-ray lasers operating at wavelengths of less than 500 Angstroms have currently been demonstrated by: (a) the use of lasers delivering the pulse energies of kilojoules or hundred of joules, most typically Nd-Glass lasers, to create highly ionized plasmas from solid targets such as Selenium and (b) the use of very short (,100 ps) or ultrashort (0.03-2 ps) laser pulses with smaller energy and size than those mentioned above, but with typically Terawatt peak power.
It should also be noted that x-ray laser radiation from laser created plasmas has been generated by either one of the following two types of excitation processes.
(i) Electron Impact Excitation
In this case energetic electrons from laser created plasmas collide with ions of a certain charge state and excite these ions to create a population inversion between two excited levels of this specie. What follows as a result is common to the operation of most other lasers: the process of stimulated emission causes the radiation corresponding to the wavelength of the radiative transition that links the two levels to amplify in intensity as it travels through the medium.
(ii) Electron Ion Recombination
A laser created plasma rapidly cools at the end of the excitation pulse by adiabatic expansion, radiation or electron heat conduction. The rapid decrease in the plasma temperature causes the electrons and ions from the plasma to recombine, creating population inversion between excited levels of the lesser charge ions that result from the recombination process. Again radiation with a photon energy corresponding to the energy difference between the inverted levels is amplified.
An important aspect of the difficulty in constructing x-ray lasers is the requirement of a powerful energy source capable of depositing very high power densities. In all the methods described above a laser is typically used as such an energy source. Higher power density deposition of lasing material is achieved by focussing the laser beams with lenses or mirrors and hence they are used to generate plasmas that are used in the ways described above to produce the amplification of soft x-ray or extreme ultraviolet radiation.
In the particular case of soft x-ray lasers, they are traditionally generated utilizing very large power infrared, visible or ultraviolet lasers to generate the medium that will produce the soft x-ray beam. In this method of generating soft x-ray/extreme ultraviolet laser radiation, electricity is used to excite a laser emitting infrared, visible or ultraviolet light. This laser beam is then used as the energy source to excite the soft x-ray/extreme ultraviolet laser. These soft x-ray lasers are not only very costly and complex, but also in some cases can only produce laser pulses at rates of a few times per hour. Many applications require the development of soft x-ray and extreme ultraviolet lasers that are simple to operate, small in size, affordable, and that can be fired repetitively.